Adaptive Voltage Converter

ABSTRACT

An adaptive voltage converter adapted to compensate for the exponential sensitivities of sub-threshold and near-threshold circuits. The converter can change its power/performance characteristics between different energy modes. The converter may comprise two or more voltage converters/regulators. A multiplexing circuit selects between the outputs of the several converters/regulators depending on the state of a control signal generated by a control facility. The converter is specially adapted to change the output of each converter/regulator based on a number of variables, including, for example, process corner, temperature and input voltage.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following:

-   1. Provisional Application Ser. No. 62/066,218, filed 20 Oct. 2014    (“Parent Provisional”); and-   2. US Application No. [Docket No. JAM009], filed simultaneously    herewith (“Related Application”).

This application claims priority to the Parent Provisional, and herebyclaims benefit of the filing date thereof pursuant to 37 CFR§1.78(a)(4).

The subject matter of the Parent Provisional and the RelatedApplication, each in its entirety, is expressly incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an adaptive voltage converter for usewith sub-threshold and near-threshold circuits.

2. Description of the Related Art

In general, in the descriptions that follow, the first occurrence ofeach special term of art that should be familiar to those skilled in theart of integrated circuits (“ICs”) and systems will be italicized. Inaddition, when a term that may be new or that may be used in a contextthat may be new, that term will be set forth in bold and at least oneappropriate definition for that term will be provided. In addition,throughout this description, the terms assert and negate may be usedwhen referring to the rendering of a signal, signal flag, status bit, orsimilar apparatus into its logically true or logically false state,respectively, and the term toggle to indicate the logical inversion of asignal from one logical state to the other. Alternatively, the mutuallyexclusive boolean states may be referred to as logic_0 and logic_1. Ofcourse, as is well known, consistent system operation can be obtained byreversing the logic sense of all such signals, such that signalsdescribed herein as logically true become logically false and viceversa. Furthermore, it is of no relevance in such systems which specificvoltage levels are selected to represent each of the logic states.

Hereinafter, reference to a facility shall mean a circuit or anassociated set of circuits adapted to perform a particular functionregardless of the physical layout of an embodiment thereof. Thus, theelectronic elements comprising a given facility may be instantiated inthe form of a hard macro adapted to be placed as a physically contiguousmodule, or in the form of a soft macro the elements of which may bedistributed in any appropriate way that meets speed path requirements.In general, electronic systems comprise many different types offacilities, each adapted to perform specific functions in accordancewith the intended capabilities of each system. Depending on the intendedsystem application, the several facilities comprising the hardwareplatform may be integrated onto a single IC, or distributed acrossmultiple ICs. Depending on cost and other known considerations, theelectronic components, including the facility-instantiating IC(s), maybe embodied in one or more single- or multi-chip packages. However,unless expressly stated to the contrary, the form of instantiation ofany facility shall be considered as being purely a matter of designchoice.

In the Related Application, circuits adapted to operate in thesub-threshold domain have been disclosed. Perhaps the single greatestchallenge of operating circuits in the sub-threshold domain is theexponential sensitivity of circuit parameters to manufacturing processvariations and operating temperature. Even circuits that operate atnear-threshold voltages experience near-exponential sensitivity totemperature and process. For the purposes of this description,near-threshold voltages shall be defined as comprising substantially thenear-exponential region between about V_(dd)=V_(th) and aboutV_(dd)=V_(th)+0.4 volts. These exponential sensitivities result inswitching speed and power fluctuations that are intolerable in mostapplications. It is therefore desirable to adapt the circuit underchanging process and temperature conditions to maintain constant ornear-constant performance and power.

As is known, the tuning of the voltage level with respect to temperaturemust be carried out differently in sub-threshold and near-threshold thanin super-threshold.

Super-threshold circuits have a relatively low sensitivity totemperature and process variations, and tend to operate more slowly athigher temperatures. In contrast, sub-threshold circuits haveexponential sensitivities to temperature and process, and actuallyoperate faster at higher temperatures. Consequently, to maintainconstant performance in a sub-threshold or near-threshold circuit acrosstemperature, supply voltage must increase as temperature falls anddecrease as temperature increases. Such a characteristic, which may ormay not be substantially linear, is typically calledcomplementary-to-absolute-temperature (“CTAT”).

Adjusting the supply voltage (“V_(DD)”) is considered to be one of thebest techniques for adapting power and performance under changingprocess and temperature. In sub-threshold circuits, circuit speedchanges exponentially with V_(DD). Most circuits already have integratedvoltage conversion circuitry, and this circuitry can be converted to anadaptive supply with only minimal overhead. However, what is needed isan adaptive voltage converter designed specifically to compensate forthe exponential sensitivities of sub-threshold and near-thresholdcircuits.

Another important challenge in sub-threshold and near-threshold circuitsis the extreme disparity between the power/performance requirements whena system is in an active mode and the power/performance requirementswhen a system is in a sleep mode. Voltage regulators are among the mostimportant circuit blocks in a sub-threshold or near-threshold chip, andit is extremely challenging to design a single voltage converter thatcan simultaneously meet the bandwidth requirements of active mode andthe ultra-low quiescent current requirements of sleep mode. In light ofthis, it is desirable to create a voltage converter that can adapt asthe system moves from active mode to sleep mode. What is needed is anadaptive voltage converter that can change its power/performancecharacteristics between different energy modes.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, a voltage conversion method is adapted to deliver toa load a regulated voltage having a selected one of a first currentcapability and a second current capability substantially less than thefirst current capability. In a first step, a first voltage isselectively developed having the first current capability. In a secondstep, a second voltage is selectively developed having the secondcurrent capability. In a third step, one of the first and secondvoltages is selected as the regulated voltage as a function of a currentconsumption of the load. In accordance with this method at least one ofthe first and second voltages is dynamically adjusted as a functioncomplementary to absolute temperature.

In one other embodiment, a voltage converter facility is adapted toperform the adaptive voltage conversion method.

In yet another embodiment, an electronic system comprising a voltageconverter facility is adapted to perform the adaptive voltage conversionmethod.

In one further embodiment, a computer readable medium is providedincluding executable instructions which, when executed in a processingsystem, causes the processing system to perform the adaptive voltageconversion method.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The several embodiments may be more fully understood by a description ofcertain preferred embodiments in conjunction with the attached drawingsin which:

FIG. 1 illustrates, in block diagram form, a general purpose computersystem adapted to instantiate any of the several embodiments;

FIG. 2 illustrates, in block diagram form, a typical integrated systemadapted to practice any of the several embodiments;

FIG. 3 illustrates, in block schematic form, one embodiment of anadaptive voltage converter;

FIG. 4 illustrates, in circuit diagram form, one other embodiment of areference voltage generator;

FIG. 5 illustrates, in flow diagram form, one method for operating theadaptive voltage converter;

FIG. 6 illustrates, in circuit diagram form, one embodiment of a buckconverter; and

FIG. 7 illustrates, in circuit diagram form, one embodiment of a linearvoltage regulator.

In the drawings, similar elements will be similarly numbered wheneverpossible. However, this practice is simply for convenience of referenceand to avoid unnecessary proliferation of numbers, and is not intendedto imply or suggest that identity is required in either function orstructure in the several embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Shown in FIG. 1 is a typical general purpose computer system 10.Although not all of the electronic components illustrated in FIG. 1 maybe operable in the sub-threshold or near-threshold domains in anyparticular embodiment, some, at least, may be advantageously adapted todo so, with concomitant reductions in system power dissipation. Inparticular, in recently-developed battery-powered mobile systems, suchas smart-phones and the like, many of the discrete components typical ofdesktop or laptop devices illustrated in FIG. 1 are integrated into asingle integrated circuit chip.

Shown in FIG. 2 is a typical integrated system 12 comprising, interalia, reference voltage (“V_(Ref)”) generator 14, reference current(“I_(Ref)”) generator 16, several digital modules, and several analogmodules. An example of an analog module is analog to digital converter(“ADC”) 18. Reference voltage generator 14 and reference currentgenerator 16 are each common modules for supplying a stable reference tosuch analog modules. Reference voltage generator 14 is sometimes used toderive the output reference current provided by reference currentgenerator 16. Also, reference voltage generator 14 and reference currentgenerator 6 may be used to supply a stable reference to modulesthroughout integrated system 12.

For convenience of reference, in the system illustrated in FIG. 2, oneinstantiation of the voltage converter 20 is illustrated. In general,the voltage converter 20 is adapted to deliver to a load, e.g., any ofthe several components comprising system 12, a regulated voltage havinga selected one of a first current capability and a second currentcapability substantially less than the first current capability. Inaccordance with the method at least one of the first and second voltagesis dynamically adjusted as a function complementary to absolutetemperature.

Shown in greater detail in FIG. 3 is one embodiment of the adaptivevoltage converter 20. A battery 22 supplies a voltage, V_(Bat), toconverter 20, which generates a lower regulated voltage, V_(Reg), thatmay be delivered to a load circuit 24, which can be a circuit of anytype. The voltage V_(Reg) may be sub-threshold, near-threshold orsuper-threshold. In general, the converter 20 may comprise two or morevoltage converters/regulators 26, of which only two are illustrated inFIG. 3. A multiplexing circuit 28 selects between the outputs of theseveral converters/regulators 26 depending on the state of a controlsignal 32 generated by a control facility 34. Control 34 alsoselectively enables and/or disables each of the converters/regulators26; and a voltage reference V_(Ref) generator 36. In addition to basicvoltage regulation, the converter 20 may be adapted to change the outputof each converter/regulator 26 based on a number of variables,including, for example, process corner, temperature and input voltage.Details of important elements and variants of the invention willdescribed below, as will a specific implementation.

As has been noted, in the embodiment illustrated in FIG. 3, converter 20comprises two converters: a buck converter 26 a for high-efficiencyconversion during active mode, and a linear voltage regulator 26 b forultra-low quiescent current operation during sleep mode. If one of theconverters 26 is in use, it would be typical to power down the otherunused converter 26 to save energy. In this embodiment, the buckconverter 26 a will generally be enabled when the system 12 is in activemode with loads on the order of 100 μA to 5 mA. In such a mode, buckconverter 26 a is capable of delivering power at a variety of voltages(including sub-threshold and near-threshold voltages) with powerefficiencies exceeding 90%. However, load currents in a sub-threshold ornear-threshold circuit may fall below 100 nA in a sleep mode, and thepower efficiency of buck converter 26 a could easily fall below 5%. Inthis sleep mode, it may be desirable to switch over to a secondconverter that offers better power efficiency. For example, linearvoltage regulator 26 b can be easily adapted to operate with quiescentcurrent on the order of 1 nA yet be capable of delivering much greatpower efficiency with load currents on the order of 100 nA. Typically,such an embodiment of linear regulator 26 b will be incapable ofsourcing active load currents in the range of 100 μA to 5 mA, butautomatic switchover to the buck converter 26 a in active mode solvesthis problem.

It is common for microcontrollers (“MCUs”) to have architected powerstates (e.g., an active state, a sleep state, a deep sleep state, etc.).Typically, such an MCU will have a power management unit (“PMU”) that isresponsible for switching between architected power states. Since thePMU is driving transitions between power states, it may also be used asthe control 34 in FIG. 3 to drive transitions between voltage converters(e.g., from buck converter 26 a to linear regulator 26 b) in theconverter 20.

As is known, transitions between voltage converters can also be drivenby components in addition to the PMU. For example, a serialcommunications interface (“SCI”) might remain active while the MCU is ina sleep state. Normally, linear regulator 26 b would be enabled becausethe system 12 is in a sleep state. However, the SCI is still active andmay require a high-performance converter like the buck converter 26 a.Consequently, it may be desirable to permit the SCI to request that thebuck converter 26 a remain powered on and selected despite the system 12transitioning to a sleep state.

Transitions between voltage converters can also be driven by currentsense circuitry. For example, if a current sensor circuit (not shown)detects that the load current has fallen below some predeterminedthreshold, then the converter 20 can switch over from buck converter 26a, which has a high load capability, to linear regulator 26 b, which hasa low load capability.

Control of the converter 20 can be achieved via software, but this maysometimes be challenging and confusing for software developers. It maytherefore be desirable to automate the transitions between voltageconverters 26 based on the architectural power state of the system andbased on the activity of peripherals in the system (e.g., the SCI).

While thus far focus has been on switching between converters 26, eachindividual converter 26[a:b] can also be designed to adapt to changingconditions and/or changing control signals. For example, buck converter26 a can change the on-time of the power switch transistor (“T_(on)”))(not shown) depending on changing input value (i.e., conversion ratio),changing load current, a changing control signal, or a variety of otherinputs. In contrast, linear regulator 26 b can adapt the tail current ofits main amplifier (not shown) depending on its load current, a changingcontrol signal, or a variety of other inputs.

In some implementations, the converter 20 may not contain two separateconverters 26. It may instead contain a single voltage converter 26 thatadapts to the power state of the load circuit. For example, linearregulator 26 b may be adapted to use a relatively high tail current of 1μA in active mode to ensure adequate bandwidth for large active modeloads, while in sleep mode regulator 26 b may be reconfigured to use alower tail current of 1 nA.

The multiplexor 28 in FIG. 3 can be implemented using any knownmultiplexing technique including a pass transistor multiplexor, atransmission gate multiplexor, a wired-OR multiplexor implementation,and any of a variety of other known embodiments. The multiplexor 28 mayalso be designed to accommodate different timing relationships duringswitch-over from one converter 26 to another. The multiplexor 28 canpass the outputs of two converters 26 at once during switch-over fromone converter to another to ensure a reliable output voltage.Conversely, the multiplexor 28 can block both converters 26 for a periodof time when switching from one to another. The multiplexor 28 can alsoswitch instantaneously from one to another.

It is typical for the converters 26 in FIG. 3 to share off-chip andon-chip passives (e.g., compensation capacitors) to save cost and area,though this is not required. In addition, converters 26 will generallyshare an input voltage, though they may also have different inputvoltages. Although converters 26 will generally provide the same outputvoltage, this is not required. For example, buck converter 26 a used inactive mode might provide a low voltage to minimize power while activelyawake; but linear regulator 26 b used in sleep mode might allow voltageto float higher since voltage level is not as important in sleep mode.Similarly, the converters 26 can have different compensation behavior.Thus, in the previous example, buck converter 26 a might be temperaturecompensated while linear regulator 26 b might not be temperaturecompensated at all.

As previously mentioned, an adaptive voltage supply is one of the bestavailable tools to manage the exponential sensitivities of sub-thresholdand near-threshold circuits. In one embodiment of the adaptive voltageconverter 20, the V_(Reg) voltage level is adjusted in response todifferent manufacturing process variations or environmental conditions.The tuning of the voltage level can be software controlled or can becontrolled by a control circuit in a closed-loop fashion. The voltagelevel output by a particular converter 26 can be controlled by changingthe reference voltage, the voltage converter gain, or any otheravailable tuning parameter with respect to temperature and/or process.

In one embodiment, the V_(Reg) voltage is dynamically adjusted as afunction complementary to absolute temperature. Although the generationof such a function can be achieved in an open-loop manner with softwarethat periodically measures temperature with a sensor (not shown), it maybe desirable to construct a closed-loop circuit that requires nosoftware intervention. For example, two-transistor sub-thresholdreference voltage generator 36′ shown in FIG. 4 can be easily adapted toadjust the V_(Reg) voltage as a function complementary to absolutetemperature simply by changing the size of one of the transistors M1-M2.Such an embodiment, used in the converter 20, automatically forcesV_(Reg) to vary as a function complementary to absolute temperaturewithout software intervention.

The tuning of the voltage level with respect to variations in themanufacturing process is also important. In general, this tuning step isbest done at the time of production test. Tuning parameters can bestored in an on-board non-volatile memory (not shown) and then loadedupon powering up for the first time. For chips that exhibit slow processcharacteristics (e.g., high threshold voltage or long gate length), theregulated voltage will generally be adjusted to a higher level to ensurea minimum performance level. Conversely, the regulated voltage willgenerally be adjusted down to save energy while maintaining performancefor chips with faster process characteristics. Any known trimmingalgorithm may be used for determining the correct voltage levelsettings.

Though this discussion has focused mainly on the adaptation of supplyvoltage in response to temperature and process fluctuations, V_(Reg) canalso be adapted to other factors. For example, as the system's workloadchanges, V_(Reg) can be changed accordingly. The system might remain ina sub-threshold or near-threshold low performance, low energy mode whilehandling background tasks like sensing and data movement. When handlingapplications with real-time requirements, the system might automaticallyincrease voltage to a super-threshold voltage to achieve higherperformance at the expense of energy efficiency.

Many of the aforementioned characteristics rely on tuning of the system12 to minimize variations across process and temperature. This requirescareful calibration at the time of post-manufacturing test.Manufacturing test requires a means to read out each important voltage.including reference voltages, internal nodes of feedback dividers, andregulated outputs. This is typically achieved by having an on-chipmultiplexer (not shown) with a buffering amplifier (e.g., see FIG. 7 inthe Related Application) that can alternately select each voltage ofinterest. Manufacturing test also employs a means to store calibratedvalues in a non-volatile manner on chip. This can be achieved usingflash memory, one-time programmable memory, fuses, or any other means ofnon-volatile data storage. Any known trimming algorithm or method may beused to set the correct calibration settings for all previouslydiscussed trimmable elements.

In accordance with the switch-over method illustrated in FIG. 5, whenthe load circuit 24 initially switches into the active mode, V_(Reg) issourced by buck converter 26 a with high power efficiency and highcurrent drive capability. When the load circuit 24 thereafter switchesinto sleep mode, control 34 powers up linear regulator 26 b, whichexhibits excellent low current drive capability and extremely lowquiescent current. After giving the regulator 26 b sufficient time towarm up, control 34 switches multiplexer 28 to source the load currentfrom the linear regulator 26 b. After a short delay, the control 34powers down the buck converter 26 a. Similarly, when the load circuit 24switches into active mode, control 34 powers up buck converter 26 a.After giving buck converter 26 a time to warm up, control 34 switchesmultiplexer 28 to source the load current from the buck converter 26 a.After a short delay, control 34 powers down the linear regulator 26 b.

In one embodiment, V_(Ref) generator 36 is tuned to have a lowtemperature coefficient (“TC”) (i.e., a near-zero coefficient). In thisembodiment, control 34 provides a first tuning parameter 38 a to tunethe absolute value of the reference across process; and a second tuningparameter 38 b to tune out process variations in the temperaturecoefficient of the voltage reference.

As illustrated in the embodiment illustrated in FIG. 3, buck converter26 a receives multiple control signals from control 34: a first signal40 a controls the absolute value of the feedback network's divide ratio;a second signal 40 b controls the temperature co-efficient of thefeedback divide ratio; and a third signal 40 c controls whether buckconverter 26 a is powered up or powered down. A more detailed diagram ofthe buck converter 26 a is shown in FIG. 6. The buck converter 26 a usesa pulse frequency modulation scheme with transistors operating in a mixof sub-threshold, near-threshold and super-threshold regimes. Thefeedback network 44 a is a resistive divider that is highly tunable,with a current source adapted to provide a bias current as a functionthat is positive to absolute temperature (“PTAT”). As previously stated,the first signal 40 a is used to control the divide ratio to combatprocess variations, and the second signal 40 b is used to control thetemperature coefficient; both the first signal 40 a and the secondsignal 40 b are typically set during IC manufacturing, but, if desired,may be changed dynamically to adapt to current operating conditions. Thetemperature co-efficient is selected to ensure that V_(Reg) varies as afunction complementary to absolute temperature, thereby ensuring thatsub-threshold and near-threshold circuits have operating speeds thatremain substantially constant across temperature. The feedback network44 a, and, in particular, the PTAT current source, includes activeamplifiers to ensure tunability of the temperature co-efficient. Onesuitable PTAT current source is the band-gap reference circuit shown inFIG. 11.18 on page 391 of Design of Analog CMOS Integrated Circuits, B.Razavi, McGraw Hill 2001.

As illustrated in the embodiment illustrated in FIG. 3, linear regulator26 b receives multiple control signals from control 34: a first signal42 a controls the absolute value of the feedback network's divide ratio;a second signal 42 b controls the temperature co-efficient of thefeedback divide ratio; and a third signal 42 c controls whether linearregulator 26 b is powered up or powered down. A more detailed diagram ofthe linear regulator 26 b is shown in FIG. 7. The linear regulator 26 buses an amplifier biased in the sub-threshold region with tail currentmuch less than 1 nA. Like the buck converter 26 a, the linear regulator26 b has a feedback network 44 b that is a highly tunable resistivedivider. The first signal 42 a is used to control the divide ratio tocombat process variations, and the second signal 42 b is used to controlthe temperature coefficient of the feedback divide ratio. The feedbacknetwork 44 b includes active amplifiers to ensure tunability of thetemperature co-efficient. This feedback network 44 b can be shared withthat of the buck converter 26 a to save area and power. As in the caseof the buck converter 26 a, the temperature co-efficient is selected toensure that V_(Reg) varies as a function complementary to absolutetemperature.

Although described in the context of particular embodiments, one ofordinary skill in this art will readily realize that many modificationsmay be made in such embodiments to adapt either to specificimplementations.

Thus it is apparent that an adaptive voltage converter designedspecifically to compensate for the exponential sensitivities ofsub-threshold and near-threshold circuits has been disclosed. Thisadaptive voltage converter is also adapted to change itspower/performance characteristics between different energy modes.Further, this method and apparatus provides performance generallysuperior to the best prior art techniques.

What is claimed is:
 1. A voltage conversion method adapted to deliver toa load a regulated voltage having a selected one of a first currentcapability and a second current capability substantially less than thefirst current capability, the method comprising the steps of: [1.1]selectively developing a first voltage having the first currentcapability; [1.2] selectively developing a second voltage having thesecond current capability; and [1.3] selecting as the regulated voltageone of the first and second voltages as a function of a currentconsumption of the load; wherein at least one of the first and secondvoltages is dynamically adjusted as a function complementary to absolutetemperature.
 2. The method of claim 1 wherein at least one of the firstand second voltages is a selected one of sub-threshold andnear-threshold.
 3. The method of claim 1 wherein both the first and thesecond voltages are dynamically adjusted as a function complementary toabsolute temperature.
 4. The method of claim 1 wherein the first voltageis developed from a reference voltage using a switching conversiontechnique.
 5. The method of claim 1 wherein the second voltage isdeveloped from a reference voltage using a linear conversion technique.6. The method of claim 1 wherein: the first voltage is developed from areference voltage using a switching conversion technique; and the secondvoltage is developed from a reference voltage using a linear conversiontechnique.
 7. The method of claim 1 wherein both the first and secondvoltages are developed from a reference voltage using a linearconversion technique.
 8. The method of claim 1: wherein the load isadapted to operate in a selected one of two current consumption modes;and wherein step [1.3] is further characterized as selecting as theregulated voltage one of the first and second voltages as a function ofthe operating mode of the load.
 9. A voltage converter facilityconfigured to perform the method of any preceding claim.
 10. Anelectronic system comprising a voltage converter facility according toclaim
 9. 11. A computer readable medium including executableinstructions which, when executed in a processing system, causes theprocessing system to perform the steps of a method according to any oneof claims 1 to 8.